New research has strengthened the possibility that biological life may exist beneath the surface of Mars. Scientists from Imperial College London have shown that the level of methane on the Red Planet can't be explained by meteorites in the atmosphere. This leaves the theory that microorganisms are producing methane gas as a by-product of their metabolic processes as one of the remaining possible explanations.

Methane consists of four atoms of hydrogen bound to a carbon atom and is the main component of natural gas on Earth. It's of interest to astrobiologists because organisms release much of Earth's methane as they digest nutrients. It has a short lifetime of just a few hundred years on Mars because it is constantly being depleted by a chemical reaction in the planet's atmosphere, caused by sunlight.

So how is the methane on Mars being replenished? One theory is that it comes from meteorites burning up in the atmosphere, but the new research shows that the volumes are too low compared with the current atmospheric levels of methane on Mars. Previous studies have also ruled out the possibility that the methane is delivered through volcanic activity.

This leaves two chief suspects - methane is being produced as a by-product of reactions between volcanic rock and water or there are microorganisms living far below the planet's surface where liquid water - a prerequisite for life - may exist.

Co-author, Professor Mark Sephton, Department of Earth Science and Engineering at Imperial College London says the work is a big step forward.

“The list of possible sources of methane gas is getting smaller and excitingly, extraterrestrial life still remains an option,” Mr Sephton said. “Ultimately the final test may have to be on Mars.”

The team say their study will help NASA and ESA scientists who are planning a joint mission to the red planet in 2018 to search for the source of methane. The researchers say now that they have discovered that meteorites are not a source of Methane on Mars, ESA and NASA scientists can focus their attention on the option of microorganism life.